This invention is related to save filters, and, more particularly, this invention is related to a high pass filter network or coupler tuned to create a desired resonance with a railroad track for coupling AC signals above a predetermined frequency around a set of insulated joints.
The high pass coupler of the invention has particular utility when used in conjunction with train detection equipment that operates with a relatively high frequency AC electric signal, for example in the frequency range of 100 to 3000 Hz, such as the detector system disclosed in U.S. Pat. No. 3,850,390, issued on Nov. 26, 1974. Such a detector system transmits an AC signal in a monitored railroad track, and the presence and/or the approach of a train on such monitored track will act as a shunt to attenuate the AC signal in a manner such that the train will be safely detected by the system. Upon detecting a train, such detector system may operate a typical signal relay to drop gates or to energize signal lights at a grade crossing or may provide a control to block signal systems, etc.
The relatively high frequency signals employed in such detector systems may not be the only electric signals in a given section or length of railroad track. In fact, it is common to find DC signals and/or relatively low frequency AC signals, say at 60 Hz, commonly used for code signaling, in the track from other equipment coupled thereto. Moreover, it is also common to use insulated joints in the railroad track to prevent transmission of electric signals from one section of track to another.
In the past tuned shunts, including a series connected capacitor and inductor, respectively coupled across each of the insulated joints in a set of joints or broad band shunts, including respective capacitors coupled across each of the insulated joints, have been used for coupling desired AC signals around insulated joints in a railroad track, but each of these shunts usually also coupled too much impedance into the track circuit, which is undesirable, especially when the track is to carry signals from movement detector systems. Also, capacitor broad band shunts still may pass certain AC signals as well as spikes or other signal noise on the track.
To signal systems coupled to a railroad track or the like, such as the signal system disclosed in the mentioned patent, the lumped impedance characteristic of the track in a monitored approach, for example, to an island at a grade crossing will vary generally logarithmically with respect to distance as viewed from the island to a beginning of approach shunt, say 3000 feet down the track. For proper operation it is desirable that the detector system monitoring the approach see such a normal lumped track impedance characteristic over the entire monitored approach, track section or the like.